The high endogenous urease activity of Campylobacter pylori was exploited in a non-invasive test for the presence of this organism in the stomach. When 13C-urea was administered orally after a test meal, urea-derived 13CO2 appeared in the respiratory CO2 of infected individuals at a constant rate for greater than 100 min. The test was validated in 26 individuals who underwent both the 13C-urea breath test and endoscopic biopsy of the antral mucosa for culture and histological examination. Each positive breath test proved to be correlated with a positive culture or Warthin-Starry silver stain of a mucosal biopsy specimen, or both.